It is known that packings produced by the method specified above are wound around pump shafts or valve rods within a stuffing box to reduce or eliminate liquid losses. The packing thus arranged is compressed by a gland with a predetermined force so that it presses against the surface of the shaft or rod.
As a result of the curvature imposed by the winding, the elementary yarns on the extrados surface of the packing are in traction while the elementary yarns on the intrados surface are compressed so that the cross-section of the packing, which initially is square or rectangular, deforms into an isosceles trapezium with its larger base on the intrados side.
As a result of this deformation, the compressive force is not transmitted uniformly through the packing. Thus it is necessary to increase the force to achieve adequate hydraulic sealing at the expense of greater wear of the packing itself due to yielding and friction.
WO-A-94/11555 discloses a packing which has a cross-section in the form of an isosceles trapezium with its larger base on the extrados side. The deformation induced by the winding makes the section square or rectangular, improving the uniformity of transmission of the compressive forces.
The elementary yarns in such a packing are however subject to yielding as in conventional packing and are, in any case, under traction and compression. Moreover, in pumps, the friction is increased since the elementary yarns on the intrados surface are pressed together by the overlying yarns and pressed onto the surface of the shaft with a consequent loss of energy and overheating of the packing.